Products made of various materials such as synthetic resin, metal, and glass and the like are often provided with surface protection materials to cover them to prevent them from being scratched or stained during processing, transportation, or storage. Among other surface protection materials, the protective film is used most widely and, in general, it is in the form of a laminated film containing a support base and an adhesive layer formed thereon. The adhesive layer is combined with an adherend so that the support base acts as a cover to protect its surface.
In recent years, liquid crystal displays and touch panel devices have been widespread, and they are composed of many members such as various optical sheets made of synthetic resins. Such optical sheets have to be highly resistant to defects such as optical distortion and, accordingly, laminated films are frequently used as protective films to prevent scratches and stains that may lead to such defects.
Those laminated films used as protective films should meet various requirements. For example, they should be unable to be removed easily from the adherends when they are exposed to changes in temperature, humidity, or their environmental factors or receive small stresses, and they should not leave part of the adhesive or adhesive components on the adherends when removed therefrom.
Of the above optical sheets, members with surface irregularities such as diffusion plates, prism sheets, and variously shaped diffusion faces formed on the back of prism sheets sometimes fail to develop a required strength of adhesion to a laminated film immediately after being combined together, leading to removal therefrom, because the adhesive layer cannot move flexibly in conformity to the irregularities. Known solutions to this problem include the use of a softened adhesive layer and the use of a tackifier to increase the adhesive strength (for example, Japanese Unexamined Patent Publication (Kokai) Nos. 2005-298630, 2012-77244 and 2012-111793).
For an adherend having soft irregular portions, however, it is sometimes impossible to obtain a required contact area, leading to an insufficient adhesive strength, only by adjusting the softness of the adhesive layer as proposed in JP '630 or JP '244. If an adequate contact area is realized by extremely softening the adhesive layer, the contact area tends to increase over time to cause an excessive increase in the adhesive strength, often resulting in difficulty in its removal or a significant amount of the adhesive being left on the adherend after its removal.
JP '793 proposes a method to control the adhesive strength by adjusting the content of the tackifier in the adhesive layer. The addition of a tackifier in large amounts can allow an adequate adhesive strength to develop even when the contact area is small immediately after combining the surfaces. However, the tackifier tends to bleed out of the surface of the adhesive layer over time or during storage under high-temperature, high-pressure conditions, leading to contamination of the adherend or difficulty in removal from the adherend due to an increase in adhesive strength.
Japanese Unexamined Patent Publication (Kokai) No. 2006-335000 proposes a laminated film designed to protect the prism face of an optical prism sheet. That proposal, however, focuses only on the shear storage modulus of the adhesive layer and, accordingly, fails to realize a suitable adhesive strength stably for various adherends with different adherend shapes or surface characteristics, sometimes leading to problems such as an excessive increase and variation over time in the adhesive strength to the back faces of various optical prism sheets with different surface forms.
WO 2010/029773 and Japanese Unexamined Patent Publication (Kokai) No. 2011-16354 also propose laminated films designed to protect the prism faces of optical prism sheets. Those proposals deal with characteristics control by controlling the block structure of the block copolymer used, and focuses also on the loss factor (tan δ) peak temperature range of adhesive layers and their shear storage modulus at 70° C. However, optical prism sheets vary in back face shape and characteristics and it is difficult for a protective film having good characteristics in a particular temperature range to serve widely for a variety of adherends. Furthermore, the proposed adhesive layers are too high in adhesive strength to serve for the back faces of prism sheets.
It could therefore be helpful to provide a laminated film that serves for protection and storage of adherends in a suitable range of adhesive strength, such adherends including surfaces of various optical sheets such as back faces of prism sheets that vary in shape and composition.